JP3658018B2 - Insulation structure for cable - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an insulating structure for medium-pressure, high-voltage, and ultrahigh-voltage cables that pass direct current or alternating current.
[0002]
[Prior art]
These cables are generally composed of a conductive core surrounded by a coaxial insulating structure. The structure includes at least one semiconductor first layer disposed in contact with the cable core, which is itself surrounded by an electrically insulating second layer, the latter being covered by a semiconductor third layer. Has been. Other outer layers help protect the cable.
[0003]
Insulating layers are usually based on high or low density polyethylene, reticulated polyethylene, or ethylene / propylene / diene terpolymers with a methylene backbone (EPDM).
[0004]
The semiconductive layer is generally composed of a polar substrate, most often a copolymer of ethylene and alkyl acrylate, loaded with carbon black. The charge varies depending on the nature of the carbon black used. For acetylene black or furnace black, the charge ratio is generally comprised between 28% and 40%.
[0005]
[Problems to be solved by the invention]
The dielectric strength of such a cable greatly depends on the quality of the interface between the semiconductive layer and the insulating layer. If the roughness at the interface level is small, the electric field is enhanced, and there is a possibility that the insulating layer is broken and a hole is formed.
[0006]
In order to obtain as smooth an interface as possible during extrusion, the substrate of the semiconductive layer of currently commercially available high voltage cables generally has a high fluidity coefficient of about 17 or a high “melt index” (high “melt melt index”). "Index" is a label indicating the presence of low molecular weight, which is measured by ASTM reference D1238 or NFT 51-016 standard), based on polymers with a very broad molecular weight distribution. However, in the insulating layer, it has been confirmed that space charges are generated near the semiconductive layer, and the accumulation thereof deteriorates the dielectric strength of the insulator and may lead to destruction.
[0007]
Some semi-conductor manufacturers use non-polar substrates based on ethylene copolymers (EPR: ethylene propylene thermoplastic elastomer or EPDM: ethylene propylene diene terpolymer with methylene backbone). To these, an oil or a plasticizer is added to easily obtain a good surface state of the semiconductive layer. These oils or plasticizers diffuse in the insulating layer and form regions with the lowest dielectric strength at the interface level where the electric field between the semiconductor layer and the insulating layer is highest.
[0008]
It is an object of the present invention to obtain an insulating structure for medium-, high-voltage, and ultra-high-voltage cables through which a direct current or an alternating current passes, which exhibits an insulation characteristic that is more stable in time than known insulation characteristics to date. To do.
[0009]
[Means for Solving the Problems]
An object of the present invention is for a cable comprising at least one semiconductor first layer adjacent to and coaxial with a cable core, said first layer being surrounded by an electrically insulating second layer, An insulating structure in which the two layers themselves are covered by a semiconductive third layer. The semiconductive layer is composed exclusively of a substrate containing a nonpolar polymer, and the components of the nonpolar polymer are insulating structures having a molecular weight of 1000 or more and a conductive charge.
[0010]
The component of the substrate preferably has a molecular weight of 5000 or more.
[0011]
When the semiconductive layer includes a compound or additive having a low molecular weight such as an oil agent or a plasticizer, these compounds move in the insulating layer. This phenomenon can result in the formation of space charge resulting in an electric field enhancement and ultimately breakdown. The enhancement of the electric field is related to the amount of charge formed, but is also related to its mobility. That is, evenly distributed charge does not provide as much electric field enhancement as localized charge. This movement can occur during cable construction or cable operation.
[0012]
Employing a semiconductive layer of the present invention composition containing only a high molecular weight compound prevents space movement within the insulating layer and thereby prevents space charge accumulation near the interface.
[0013]
In a first variation, the substrate is polyethylene, polypropylene, polystyrene, and copolymers thereof, a composite of polyethylene, polypropylene, polystyrene, and a polymer selected from these copolymers, and polyethylene, polypropylene, polystyrene, and It is selected from a mixture of compounds selected from these copolymers and the composite.
[0014]
In a second variant, the substrate is selected from polyolefin thermoplastic elastomers and mixtures thereof.
[0015]
The selection of the polymer constituting the substrate depends on the quality of the interface with the insulating layer and the mechanical properties of the resulting semiconductive layer and does not need to rely on additives.
[0016]
The present invention has an advantage of suppressing the movement of a compound having a small molecular weight and stabilizing the insulating characteristics of the insulating structure. As a result, the quality of the interface between different layers is less critical as a parameter.
[0017]
The charge is carbon black that contains as little impurities as possible. Furnace black or “KETJEN” black can be used, but it is preferred to choose very pure acetylene black.
[0018]
In some cases, the substrate further comprises a reticulating agent. After molding the material by extrusion, it can be reticulated from the viewpoint of improving thermodynamic properties. These characteristics are particularly important for cables carrying alternating current.
[0019]
Other features and advantages of the present invention will become apparent upon reading the following examples, which are of course illustrative and non-limiting, with reference to the accompanying drawings.
[0020]
【Example】
A pressure wave test was performed using the apparatus shown in FIG. This test can evaluate the enhancement of the electric field in the insulating structure.
[0021]
A sample 1 of an insulating structure for pressure wave test is shown in a top view in FIG. 2 and a cross-sectional view in FIG. On a circular surface with a diameter A of 20 mm,
A semiconductor first layer 2 having a thickness B of 0.5 mm,
An electrically insulating second layer 3 with a thickness C of 0.8 mm,
The same semiconductor third layer 4 as layer 2;
Are placed on top of each other.
[0022]
The apparatus of FIG. 1 consists of a laser 10 “YAg” whose laser beam is applied to a target 11 corresponding to sample 1 and each semiconductor of the target constitutes an electrode (+) and (−). The laser beam absorbed on the surface of the electrode 2 (−) decomposes the surface by thermal decomposition, and the generated gas causes a pressure wave across the sample. This wave modulates the charge image on the electrode and gives access to the volume charge density in the sample.
[0023]
The photodiode 12 allows the detector 13 to be tuned to the laser 10. The circuit is fed by a high voltage feed line with a resistor 15. The recorded data is transferred for processing by the computer 16 and displayed on the graphic register 17 as a function of time. The laser 10 sends a wave to the target 11 causing the appearance of space charge and the change in electric field distribution, which is then measured by the detector 13.
[0024]
Example 1
A sample of a prior art insulation structure similar to the sample shown in FIG. 2 is created.
This sample
A semiconductive first layer comprising a polar substrate based on a copolymer of ethylene and an alkyl acrylate, wherein the “melt index” of the copolymer has a value of 8 and its ester The first layer, wherein the content is 20%, to which acetylene black charge is added in a ratio of 66 parts by weight to 100 parts by weight of the substrate;
An electrically insulating second layer comprising an olefin thermoplastic elastomer;
-Includes the same semiconductive third layer as the first layer.
[0025]
Thus, this sample is subjected to a pressure wave test using the apparatus shown in FIG. It can be seen that negative charges appear at the cathode 2 in a significant proportion. Therefore, the electric field is strengthened by 20% or more, and electric charges remain trapped in the material for several hours after the power is cut off.
[0026]
Example 2
A sample of a prior art insulation structure similar to the sample shown in FIG. 2 is created. This sample
A semiconductive first layer comprising a polar substrate based on a copolymer of ethylene and an alkyl acrylate, wherein the “melt index” of the copolymer has a value of 8 and its ester The first layer, the content of which is 20%, to which acetylene black charge was added in a ratio of 66 parts by weight to 100 parts by weight of the substrate;
An electrically insulating second layer made of chemically reticulated polyethylene (PRC);
-Including the same semiconductive third layer as the first layer.
[0027]
The sample is then subjected to pressure wave testing using the apparatus shown in FIG. It can be seen that a significant amount of negative charge appears near the cathode 2 and that the charge remains trapped in the substrate after power interruption. The electric field enhancement is 20% or more.
[0028]
Example 3
A sample of an insulating structure according to the present invention similar to the sample shown in FIG. 2 is made. This sample
A semi-conductor first layer consisting of a non-polar substrate, to which acetylene black charge is added in a ratio of 66 parts by weight to 100 parts by weight of the substrate, the substrate being a “melt index” With a molecular weight of 10 ³ to 10 ⁷ , a polyethylene (PE) 20% centered at 1.1 × 10 ⁶ and a “MOONEY” viscosity of 40 (measured according to the NFT 43005 standard). Said first layer comprising about 80% ethylene and propylene copolymer containing about 50% by weight of ethylene with a molecular weight in the range of 10 ³ to 10 ⁷ and centered at 1.2 × 10 ⁵ When,
An electrically insulating second layer made of chemically reticulated polyethylene (PRC);
-Including the same semiconductive third layer as the first layer.
[0029]
The sample is then subjected to a pressure wave test using the apparatus shown in FIG. The electric field enhancement is 10% or less, and trapped charges do not persist in the insulating material after power interruption.
[0030]
Example 4
A sample of an insulating structure according to the present invention similar to the sample shown in FIG. 2 is made. This sample
A semi-conductor first layer consisting of a non-polar substrate, to which acetylene black charge is added in a ratio of 66 parts by weight to 100 parts by weight of the substrate, the substrate being a “melt index” 20% polyethylene (PE) with a valence of 2 and a molecular weight in the range of 10 ³ to 10 ^{7 with} a molecular weight centered at 1.1 × 10 ⁶ , and a “MOONEY” viscosity (measured according to NFT43005 standard) The first layer comprising about 40% ethylene and propylene copolymer containing about 50% by weight of ethylene and having a molecular weight of about 40 and a molecular weight of 10 ³ to 10 ⁷ and centering on 1.2 × 10 ⁵ When,
An electrically insulating second layer comprising an olefin thermoplastic elastomer;
-Including the same semiconductive third layer as the first layer.
[0031]
The sample is then subjected to a pressure wave test using the apparatus shown in FIG. The electric field enhancement is 10% or less, and trapped charges do not persist in the insulating material after power interruption.
[0032]
Example 5 A sample similar to the sample described in Comparative Example 4 is prepared, but paraffin oil is added to the substrate of the semiconductive layer in a proportion of 5% by weight with respect to the substrate.
[0033]
The sample is then subjected to a pressure wave test using the apparatus shown in FIG. The field enhancement in this case is 140%.
[0034]
Of course, the invention is not limited to the described embodiments, and many variations can be made accessible to those skilled in the art without departing from the intent of the invention. In particular, all of the means can be replaced by equivalent means without departing from the scope of the invention.
[Brief description of the drawings]
FIG. 1 is an overall conceptual diagram of a pressure wave testing apparatus.
FIG. 2 is a top view of a sample insulating structure for a pressure wave test.
FIG. 3 is a schematic cross-sectional view of the sample of FIG.
[Explanation of symbols]
1 Sample 2 Semiconductor first layer, electrode (-)
3 Electrically Insulating Second Layer 4 Semiconductor Third Layer 10 Laser 11 Target 12 Photodiode 13 Detector 14 High Voltage Power Supply 15 Resistor 16 Computer 17 Graphic Register

Claims (6)

For a cable comprising at least one semiconductor first layer adjacent to and coaxial with a cable core, said first layer being surrounded by an electrically insulating second layer, the second layer itself being semiconductive An insulating structure covered by a body third layer, wherein both of said semiconductive layers are exclusively from a non-polar polymer having only a molecular weight component of 1000 or more and a conductive charge insulation structure, wherein the formed Turkey.   The structure according to claim 1, wherein the component of the substrate has a molecular weight of 5000 or more.   The substrate is selected from polyethylene, polypropylene, polystyrene, and a copolymer thereof, a polymer composite selected from polyethylene, polypropylene, polystyrene, and a copolymer thereof, and a mixture of the compounds. The structure according to claim 1 or 2, characterized in that   The structure according to claim 1 or 2, wherein the substrate is selected from a polyolefin thermoplastic elastomer and a mixture thereof.   The structure according to any one of claims 1 to 4, wherein the charge is acetylene black.   The structure according to any one of claims 1 to 5, wherein the substrate further contains a cross-linking agent.

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